Optical multiplexer and cross-switch using etched liquid crystal Fabry-Perot etalons

ABSTRACT

An LC optical multiplexer according to the present invention is a two-etalon Fabry-Perot laser etched into many (&gt;100) sub-etalons in a rectangular array. Each sub-etalon is independently tunable and can be coupled to a distinct fiber. Any single sub-etalon or random combination of sub-etalons is free to be tuned to a particular wavelength corresponding to one of the input channels. This allows for any combination of signals (i.e digital video, data and voice) in a signal broadband channel to be switched to any of several receivers. Wavelength division multiplexing (WDM) is used to combine or separate individual types of signals from a single fiber. Phase-matching coatings are used on the materials within the Fabry-Perot gap, thereby enhancing transmission performance of the WDM device. Mechanical techniques are used to widen the Fabry-Perot gap beyond a 100-micron LC thickness. The widening permits greatly enhanced spectral discrimination (i.e. many more WDM channels) across the device response range, which is expanded to ITU standards by use of the twin etalon configuration. A fully agile optical cross-switch of many (&gt;100) transmitted and received channels is achieved by use of two multiplexers in an optical network.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of application Ser. No.09/775,970, filed Feb. 2, 2001, now pending, which claims benefit under35 U.S.C. §119(e) from provisional application No. 60/220,682, filedJul. 25, 2000. The No. 60/220,682 and Ser. No. 09/775,970 applicationsare both incorporated by reference herein, in their entirety, for allpurposes.

INTRODUCTION

[0002] The present invention relates generally to the fields of opticalnetwork communications and optical switching. More particularly, theinvention relates to multiplexing and cross switching of digital signalsin an optical communications network.

BACKGROUND OF THE INVENTION

[0003] Optical transport has become an important data channel medium.From the advent of fiber-optic long distance telecommunications in the1980's to the extensive optical fiber information distributioninfrastructure investments currently being made, there has been aninsatiable demand for the great bandwidth promised by optical transport.

[0004] Some of the workhorses of optical communications technology arethe Wavelength Division Multiplexing (WDM) multiplexers anddemultiplexers. WDM multiplexing has been used to provide multiplecommunication channels of transmission (MUX) or reception (DEMUX) withina single optical fiber carrying a broad wavelength signal. Multiplechannels of transmission or reception are accomplished by isolation ofnarrow wavelength regimes within the broad, transmitted passband. If, inaddition, the device used to select a narrow wavelength region, orchannel, can be selectively tuned to any narrow region within thepassband, then two such devices in series establish a complete opticalcross-connection (or cross-switch).

[0005] Optical WDM according to the prior art is obtained usingminiature diffraction gratings coupled to fibers or bundles of opticalfibers. The maximum number of channels currently available in a WDMsystem is about 24. Using recently developed echelle grating technology,there exists the potential to increase that to 40-80 channels. Althoughthese are impressive improvements, it would certainly be useful toproduce a WDM system with over one hundred channels. Unfortunately thatis not available in the prior art.

[0006] Basic liquid crystal Fabry-Perot (LCFP) etalon technology hasbeen known for some time. And it has been known to make an LCFP etalontunable. However, prior art optical WDM systems have not usefullyexploited LCFP etalon technology.

[0007] As a preliminary matter, the basic aspects of LCFP etalontechnology is reviewed as follows. Referring to FIG. 1, a cross sectionview of a liquid crystal-filled Fabry-Perot etalon according to theprior art is illustrated. A first etalon substrate 102 and a secondetalon substrate 104 are spaced apart from one another. The etalonsubstrates 102, 104 are typically formed of fused silica.Precision-dimensioned spacer beads 122, 124 define the spacing betweenthe etalon substrates 102, 104. Dielectric reflector layers 106, 108 arecoated onto each of respective opposed faces of the etalon substrates102, 104.

[0008] Transparent conductor layers 110, 112 are also coated onto thesubstrates 102, 104. The top coating layers on each of the substrates102, 104 are polyimide alignment layers 114, 116. After coating, thepolyimide is buffed to provide alignment functionality. A liquid crystalmaterial 130 is filled in between the substrates 102, 104.

[0009] Basic LCFP arrangements were first described in the late 1970'sby W. J. Gunning and P. Yeh. For specifics, refer to Gunning and Yeh“Multiple-Cavity Infrared Electro-Optic Tunable Filter”, SPIE Proc.,202, 21-25 (1979), and Yeh and Gunning SPIE Proc., 202, pp. 2-15 (1979).Other publications that show the subsequent development of thistechnology are Gunning et al., “A Liquid Crystal Tunable Filter: Visibleand Infrared Operation”, SPIE Proc., 268 (1981), and Maeda et al.,“Electronically Tunable Liquid-Crystal-Etalon Filter for High DensityWDM Systems”, IEEE Photonics Technology Lett., 2 No. 11, (1990).

[0010] By applying current to the conducting layers 110 and 112, theliquid crystal (aligned by the polyimide layer) changes its orientationrelative to the axis of light passed through the system, so that theindex of refraction of the material within the etalon gap iselectronically tunable. Because the wavelength of light being passed bya Fabry-Perot is a function of the refractive index in the etalon gap,the device may be scanned through wavelength or positioned to acalibrated wavelength by simple voltage tuning. This simple LCFP tunablefilter is here improved upon and adapted for simultaneous passage ofmultiple wavelengths within narrow bands in a WDM or cross-switchconfiguration.

[0011] It has been proposed to use a tunable liquid crystal Fabry-Perotetalon as a light modulator. For details, refer to U.S. Pat. No.4,779,959 to Saunders. A twisted-nematic LCFP device has been describedthat is tunable and polarization insensitive. For details refer to U.S.Pat. No. 5,068,749 to Patel. It has also been proposed to use a nematicLCFP as a tunable filter or as a light modulator. For details refer toU.S. Pat. No. 5,111,321 to Patel and U.S. Pat. No. 5,150,236 to Patel.

[0012] Tunable liquid crystal etalons, as described in the prior art,are not useful for WDM or cross-switching. There are salient limitationsof the prior art that evidence this. First, the prior art LCFP etalonshave limited spectral resolution, which limits the number of possibleWDM channels. Second, in response to the limited spectral resolution,the Patel '236 patent enhances the reflectivity of the dielectriccoatings to greater than 95%. Although this modification of the earlier,simple designs (specifically, those of Yeh and Gunning (1979), Gunningand Yeh (1979), and Gunning et al. (1981)) does enhance spectralresolution, it drastically sacrifices transmission performance. Theenhanced reflectivity produces a longer photon path-length within theFabry-Perot resonant cavity, intrinsic scattering and absorption lossesare enhanced, and thus, overall LCFP etalon transmission is reduced.Furthermore, the scanning range of the LCFP etalon is not enhanced bythe modifications taught by the Patel '236 patent, and the ultimatespectral resolution remains limited by Fabry-Perot etalon substrateparallelism and surface defects. Finally, the prior art does notexplicitly detail the design of a multiple channel WDM device or across-switch.

[0013] The spectral resolution of a Fabry-Perot filter is determined bythe thickness of the resonant gap between the etalon reflecting layers.The attainable resolution is given by

Δλ=(λ²\2μt)\F  (1)

[0014] where λ is the sampled wavelength, μ is the refractive index ofthe material in the etalon gap, and t is the gap thickness. F is aquantity commonly called the finesse. Finesse depends upon thereflectivity of the dielectric coatings, the parallelism of thereflecting surfaces, and upon optical defects in the etalon glasssubstrates or in the medium between the plates. F is typically a valuebetween 8-50 for plane parallel Fabry-Perot etalons.

[0015] A Fabry-Perot etalon using liquid crystal in the gap (accordingto the prior art) is limited to a maximum etalon gap (t) of only 30-100microns, depending on the LC used. Larger gap spacings are not possiblefor the prior art LC etalon because the liquid crystal fractures if thegap exceeds 30-100 microns. In prior art, the etalon gap spacing islimited by the effective limit of the LC layer thickness. Thus, a liquidcrystal etalon designed to permit larger gap spacing and hence improvedspectral resolution is needed to allow the largest possible number ofWDM channels.

[0016] It has been proposed to enhance the gap of an LCFP etalon for usein WDM. However, this proposal suffers from the disadvantages that itdoes not address cross-switching capability by parsing out the realestate of the transparent conducting layer, and that the design of thegap enhancement is mechanically unstable. For additional details, referto U.S. Pat. No. 5,321,539 to Hirabayashi et al.

[0017] The International Telecommunications Union (ITU) has specifiedthe wavelength bands beginning at 1528 nm (C-band) and 1884 (L-band) astelecommunications network standards (ITU standard G.692). The C-bandspecification is 1528 nm-1560 nm. In addition, the ITU has set 100channels within each band as the WDM goal standard, although that numbermay soon be changed to 200 channels.

[0018] Thus, what is needed is a device that increases the number of WDMchannels that can be isolated by existing devices, simultaneouslyestablishes the cross-connection, and does so with a mechanicallyrobust, solid-state device.

SUMMARY OF THE INVENTION

[0019] The present invention increases the number of WDM channels thatcan isolated by existing devices, simultaneously establishes thecross-connection, and does so with a mechanically robust, solid-statedevice.

[0020] It is an object of the present invention to provide an opticalcross-connection capable of handling 100 or more channels.

[0021] It is also an object of the present invention to provide a WDMmultiplexer/demultiplexer that can handle 100 or more channels.

[0022] It is another object of the present invention to use fused silicaspacer plate and spacer posts between Fabry-Perot reflectors to providean arbitrary increase in spectral resolution of a liquid crystalFabry-Perot etalon.

[0023] It is still another object of the present invention to provide asubstantial increase in transmission for a LC etalon by phase matchingof coatings in the LC cell.

[0024] It is a further object of the present invention to provide aliquid crystal Fabry-Perot etalon with phase matched coatings used onall surface interfaces.

[0025] It is an additional object of the present invention to provide anLC etalon having multiple filters, or channels, on a single glasssubstrate.

[0026] The present invention expands the number of channels to more than200, and uses an etched, liquid crystal Fabry-Perot (LCFP) as itsfundamental optical component. This invention adapts the LCFPspecifically for WDM and cross-switching, and germane prior artdescribes the LCFP. This invention is a device that provides better than0.32 nm spectral resolution in the C-band (>100 channels), and israndomly tunable to any of those spectral resolution elements (channels)in less than 10 milliseconds.

[0027] The present invention adheres to ITU standards, by provision of adevice that is able to simultaneously resolve and select fortransmission 0.5 nm wavelength slices within a 100 nm C-band or L-band.The invention is configured as an arbitrary number of wavelength tunableLCFP filters etched upon one substrate. Etching to convert a singletunable LCFP filter to multiple filters on one substrate is a feature ofthe invention. This etched etalon is placed in series with a similaretalon, such that one etalon (the resolving etalon) defines the spectralresolution, while the second etalon (the suppression etalon) suppresseshigher orders of the resolving etalon. So configured, the twin etalonsystem provides a 100 nm free spectral range, and 0.5 nm spectralresolution. The spectral resolution is enhanced relative to prior art bymechanical extension of the etalon gap—another aspect of this invention.The transmission of the twin etalon configuration is optimized by theuse of phase-matched coatings and anti-reflection coatings—a thirdaspect of the invention.

[0028] The present invention overcomes the spectral resolutionlimitation of prior art devices by design of an air-liquid crystal orglass-liquid crystal hybrid etalon gap, while the spectral scanningdistance is preserved by combination of multiple etalons in series. Assuch, the high reflectivity requirements of the prior art are avoided,and LCFP etalon transmission is not sacrificed by use of extremely highreflectivity. In addition, phase matching of reflective coatings andproper anti-reflection coatings within the etalon gap according to thepresent invention further optimize LCFP etalon transmission. The presentinvention also uses laser etching of the etalon's transparent conductinglayer to achieve a multiple channel, tunable, WDM or cross-switch upon asingle LCFP substrate.

[0029] The present invention establishes an air-liquid crystal or aglass-liquid crystal hybrid gap by mechanically enhancing the etalongap.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Additional objects and advantages of the present invention willbe apparent in the following detailed description read in conjunctionwith the accompanying drawing figures.

[0031]FIG. 1 illustrates a cross section view of a liquid crystal-filledFabry-Perot etalon according to the prior art.

[0032]FIG. 2 illustrates a perspective view of a twin etalon WDM deviceaccording to the second embodiment of the present invention.

[0033]FIG. 3 illustrates an elevation view of the twin etalon WDM deviceof FIG. 2.

[0034]FIG. 4 illustrates an end-on view (along the optical path) of theWDM device of FIG. 2.

[0035]FIG. 5 illustrates a schematic representation of two WDM devicesoperating as a transmitter (left hand side) and a receiver (right handside) to form a cross-switch according to the second embodiment of thepresent invention.

[0036]FIG. 6 illustrates a cross sectional view of a single liquidcrystal-filled Fabry-Perot etalon according to the third embodiment ofthe present invention, with an enhanced gap width implemented via ahybrid air plus LC gap.

[0037]FIG. 7 illustrates a cross sectional view of a single liquidcrystal-filled Fabry-Perot etalon according to the third embodiment ofthe present invention, with an enhanced gap width implemented via ahybrid glass plus LC gap.

[0038]FIG. 8 illustrates a schematic view of coatings that are appliedto various surfaces of a single Fabry-Perot etalon according to thepresent invention to optimize transmission when materials of differingrefractive index are present within the resonant cavity.

[0039]FIG. 9 illustrates a perspective view of a single etalon LCFabry-Perot component stressing the ability to establish an array ofindividual LC cells on a single substrate, by etching the underlyingconducting layer applied to the etalon substrate.

[0040]FIG. 10 illustrates the spectral response of a WDM etalon pairaccording to a first working example implementing a two-inch substrate.

[0041]FIG. 11 illustrates the spectral response of a WDM etalon pairaccording to a second working example implementing a three-inchsubstrate.

DETAILED DESCRIPTION OF THE INVENTION

[0042] Nematic liquid crystals have optical anisotropy, orbirefringence. They exhibit double refraction, wherein light polarizedparallel to the director has a different index of refraction than lightpolarized perpendicular to the director. The director is along the samedirection as the surface rubbing directions assuming that both thealignment surfaces are rubbed in the same directions, which is the caseaccording to the present invention.

[0043] Nematic liquid crystals also show dielectric anisotropy, whereinthe dielectric constants parallel and perpendicular to the director arenot the same. The dielectric anisotropy introduces body torque on themolecules in the presence of an external field, which in turn gives riseto the director re-orientation. Under the external field, the directorof the liquid crystal with a positive dielectric anisotropy tends to bealigned parallel to the external field, while the director of the liquidcrystal with a negative dielectric anisotropy tends to be alignedperpendicular to the external field. The liquid crystal E44, usedaccording to a preferred implementation of the present invention, has apositive dielectric anisotropy, but our the invention also encompassesthe use of liquid crystal materials with negative dielectric anisotropy.

[0044] An AC electric field on the order of 2 kHz is used in order toavoid the separation and accumulation of free ions in the liquidcrystals.

[0045] A Fabry-Perot etalon that exploits this variable index ofrefraction effect can be constructed by coating two fused silica plateswith a transparent conductor and a broadband reflective coating.Increased spectral resolution can be achieved by creating etalons withboth an LC cavity and a solid spacer or LC-solid spacer-air gapcombinations. The total possible change of index of the LC underconsideration is Δn=0.25, which corresponds to a maximum possible changeof wavelength of 158 nm (Δλ≈λΔn). Therefore, a thin layer of LC canallow tuning of many orders with a low resolution etalon, and up toseveral hundred orders with a medium to high resolution etalon.

[0046] For the perfect Fabry-Perot etalon (one with no defects) theintensity of light at the fringe plane is given by the expression,$\begin{matrix}{{I( {\delta,\lambda} )} = {{{I_{o}(\lambda)}\frac{1}{1 + {( {2{F/\pi}} )^{2}{\sin ( {\delta/2} )}}}\quad {with}\quad \delta} = \frac{4\quad d\quad {\mu\pi}\quad {\cos (\theta)}}{\lambda}}} & (2)\end{matrix}$

[0047] where λis the wavelength, I_(o) is the intensity at the center ofeach Hadinger fringe, d is the plate separation, θ is the angle measuredfrom the plate normal and μis the index of refraction of the materialbetween the etalon plates. The finesse F, of the etalon is given by,$\begin{matrix}{F = \frac{\Delta \quad \lambda}{\delta \quad \lambda}} & (3)\end{matrix}$

[0048] where Δλ is the free spectral range and δλ is the spectralresolution element, or the spectral width of the instrument function.The free spectral range (FSR) of a single Fabry-Perot etalon is givenby: $\begin{matrix}{{\Delta \quad \lambda} = \sqrt{\frac{\lambda^{2}}{2\quad \mu \quad d}}} & (4)\end{matrix}$

[0049] The angular acceptance or field of view of each order is given by$\begin{matrix}{\theta = {2\sqrt{\frac{\lambda}{d\quad F}}}} & (5)\end{matrix}$

[0050] Here λis the center wavelength of the Fabry-Perot based filter,and δλ is the full width half max (FWHM). The angular diameter of thecentral order of the Fabry-Perot can be calculated using eqn. 5. Thelinear size of an aperture designed to intercept the central orderdepends on the effective focal length (f) of the spectrometer system.The diameter of the central order at the focal plane is given by:$\begin{matrix}{D_{ap} = {2\quad f_{0}\sqrt{\frac{\lambda}{d\quad F}}}} & (6)\end{matrix}$

[0051] Thus, the linear diameter of the fringe is simply the angulardiameter of the fringe multiplied by the focal length. A dual etalonconfiguration demonstrates high resolution and broad frequency agility.Utilizing the full ring produced by the Fabry-Perot etalon allows formuch higher throughput than is available with a grating (Echelle orCzezny-Turner) spectrometer. The luminosity-resolution product (RL) of aFabry-Perot etalon is given by

RL=2π(Area)  (7)

[0052] where the area used is the area of the etalon plate. This isseveral hundred times larger than the RL product for a gratingspectrometer with the same spectral resolution, and with a grating ofequal area to that of the Fabry-Perot etalon plates. Thus, aspectrometer system based on a Fabry-Perot etalon can reduce weight andsize, while achieving spectral resolutions equal to or greater thanthose achievable with a grating system. These advantages are achieved inLC Fabry-Perot configurations with no moving mechanical parts.

[0053] The transmission of a perfect etalon is: $\begin{matrix}{T = ( {1 - \frac{A}{1 - R}} )^{2}} & (8)\end{matrix}$

[0054] where T is the transmission, A is the absorption and R is thereflectivity of the dielectric reflector coating.

[0055] Whereas a prior art LCFP etalon achieves improved spectralresolution at the expense of transmission performance (by enhancing thefinesse through the use of dielectric coatings with reflectivity greaterthan 95%), the present invention minimizes transmission losses. Thetransmission losses are minimized at a specified spectral resolution byremoving the high reflectivity requirement, and by the application ofphase-matched and anti-reflection coatings within the LCFP gap. Thus,the present invention satisfies the need for high spectral resolutionand high transmission in a WDM device.

[0056] Whereas no prior art LC etalon has the ability to perform tunableswitching of multiple wavelength channels simultaneously, as requiredfor a WDM device, the present invention creates multiple, independentlytunable cells upon on single Fabry-Perot substrate. These multiple cellsare created by laser etching of the conducting layer into multiple,distinct pixel cells. Each cell is independently tunable to individualwavelengths. The result is an all-optical, multiple wavelength switch ofsignals (either analog or digital) by wavelength division multiplexingusing a solid state tunable device.

[0057] An LC optical multiplexer according to the present invention is atwin Fabry-Perot etalon laser etched into many sub-etalons in arectangular array. Each sub-etalon is independently tunable and can becoupled to a fiber. Any single etalon or random combination of etalonscan be tuned to a particular wavelength corresponding to one of theinput channels. This allows for any combination of signals (i.e.,digital video, data and voice) in a signal broadband channel to beswitched to any of several receivers. In addition, wavelength divisionmultiplexing is used to combine or separate individual types of signalsfrom a single fiber. An LC optical cross-switch is accomplished bypositioning a first LC multiplexer to sort broadband input byprescription, and a second multiplexer to select wavelengths afterre-mix of the wavelengths defined by the first multiplexer.

[0058] One feature of the present invention is the use of phase-matchingcoatings on the materials within the Fabry-Perot gap, thereby enhancingthe device throughput by roughly 10-fold over prior art LC Fabry-Perotdevices that do not discuss this throughput limitation.

[0059] Another feature of the present invention is etching of theconducting layer material so that a single LC cell is converted intomultiple cells each acting independently. Each element is tunableindependent of other cells by independent electronic control of therefractive medium in each cell.

[0060] Still another feature of the present invention is the use ofmechanical techniques to widen the Fabry-Perot gap beyond a 100-micronLC thickness limit. That widening permits greatly enhanced spectraldiscrimination (i.e. many more WDM channels) across the device responserange.

[0061] Another feature of the invention is expansion of the scannablespectral passband by placing two LCFP etalons in series. Combination ofenhanced free spectral range and enhanced spectral resolution permitstunable selection of more than 100 spectral regions within the telecompass band defined by ICU standards. For example, an LC multiplexingsystem is useful to select just the data component of a broadbandchannel and switch that among one or several sub-networks. Thatswitching is made entirely arbitrary by use of a second multiplexer atthe distribution end of a cross-switch system.

[0062] The present invention provides the following advantages overprior art describing WDM devices and optical cross-switches:

[0063] Elimination of any need for opto-electronic conversion

[0064] Expansion of wavelength channel selectivity to more than 100

[0065] All permutations of the defined input-output wavelengths areselectable

[0066] Low power requirements

[0067] No manual tuning is required

[0068] Maximized optical throughput

[0069] Arbitrarily narrow bandpass (i.e., spectral resolution)

[0070] All solid state, mechanically robust design

[0071] A system developed according to the present invention uses twoliquid crystal Fabry-Perot (LCFP) etalons in series and has 100-200 WDMchannels in its simplest manifestation. According to alternateembodiments where the LCFP etalons are coupled with high order sortingfilters or where three etalons are configured in combination, the numberof channels is in the thousands.

[0072] A system according to the present invention is tunable, unlikestatic grating systems. Static grating systems transmit specifiedwavelengths (channels) as a spatial function of grating spacing andgrating reflection, which is fixed at manufacture. An LCFP etalon-baseddevice is free to be tuned to transmit any wavelength within its passband to a particular spatial location.

[0073] Finally, the LCFP etalon device is entirely solid state, with nomoving parts. Spectral tuning is accomplished by low current(micro-amps) electronic control of the refractive medium (liquidcrystal) between the Fabry-Perot reflectors. In this way the LCFP etalonis superior to other WDM switching devices now in use.

[0074] A first embodiment of the present invention is represented by anoptical wavelength division multiplexing device that implements aspectsof the present invention. This WDM device embodiment may be implementedvarious ways.

[0075] One implementation of the WDM device (first) embodiment is viaplacement of two LC Fabry-etalons in series, thereby establishing anoptical multiplexing device with sufficient free spectral range andspectral resolution to cover the ITU standard telecom band pass, whileretaining sufficient spectral resolution to achieve more than 100resolvable wavelength channels. The optical multiplexing device includesa first liquid crystal Fabry-Perot etalon and a second liquid crystalFabry-Perot etalon. The first and second liquid crystal Fabry-Perotetatons are optically coupled together in series with one another.

[0076] A second embodiment of the present invention is represented by anoptical cross-switch that implements aspects of the present invention.This cross-switch embodiment may be implemented various ways.

[0077] One implementation of the cross-switch (second) embodiment is viaan optical cross-switch formed by placing two WDM multiplexing devicesin series. One multiplexing device acts as wavelength selectivetransmitter, and a second acts as a simultaneous multi-wavelengthoptical receiver. Each transmitter and receiver is a multiplexingdevice, and each in turn is composed of a pair of LC Fabry-Perotetalons.

[0078] A third embodiment of the present invention is represented by aliquid crystal Fabry-Perot etalon that implements aspects of the presentinvention. This etalon embodiment may be implemented various ways.

[0079] One implementation of the etalon (third) embodiment is via aliquid crystal Fabry-Perot etalon with a resonant gap expanded bymechanical inclusion of a hybrid glass-LC spacing. The etalon has afirst substrate coated on a first side with a first transparentconductor layer, a first reflector layer being disposed over the firsttransparent conductor layer on the first side of the first substrate,and an alignment layer being disposed over the first transparentconductor layer on the first side of the first substrate. The etalonalso has a spacer plate coated on a first side with a second transparentconductor layer, the first side of the spacer plate facing the firstside of the first substrate, and the spacer plate having a second sideopposite its first side. Plural spacers are disposed between the firstsubstrate and the spacer plate to define a predetermined gap between thefirst substrate and the spacer plate, and liquid crystal is filled inbetween the first substrate and the spacer plate. The etalon furtherincludes a second substrate coated on a first side with a secondreflector layer, the first side of the second substrate facing thesecond side of the spacer plate.

[0080] A second implementation of the etalon (third) embodiment is via aliquid crystal Fabry-Perot etalon with multiple, independently tunableresonant cells established by etching of the elctronic conducting layer,and independent electronic control of the refractive index in each cell.The etalon includes a first substrate coated on a first side with afirst transparent conductor layer, a first reflector layer beingdisposed over the first transparent conductor layer on the first side ofthe first substrate, and an alignment layer being disposed over thefirst transparent conductor layer on the first side of the firstsubstrate. The etalon also includes a second transparent conductorlayer, liquid crystal filled in between the alignment layer and thesecond transparent conductor layer, and a second substrate coated on afirst side with a second reflector layer, the first side of the secondsubstrate facing the first side of the first substrate. The firsttransparent conductor layer and the second transparent conductor layerare each etched so as to form multiple independent etalons.

[0081] A third implementation of the etalon (third) embodiment is via aliquid crystal Fabry-Perot etalon with a resonant gap expanded bymechanical construction of a hybrid air-glass-LC sandwich within theresonant cell. The etalon includes a first substrate formed of glass, aspacer plate formed of glass, liquid crystal disposed between the firstsubstrate and the spacer plate, and a second substrate formed of glass,spaced apart from the spacer plate by an air gap.

[0082] A fourth implementation of the etalon (third) embodiment is viaselective design and application of anti-reflective (AR) and phasematched coatings to the various surfaces of a liquid crystal Fabry-perotetalon. An AR coating specific to the ITU telecom pass band are appliedto the initial (input) surface of the first etalon substrate. This firstAR coating minimizes scattering loss at the air-glass interface. Thereflector coating applied to the second surface (facing the resonantcavity) of the first etalon is formulated to include the second ARcoating, and it minimizes scattering loss as light passes through theglass, conducting layer, reflective layer, polyamide, LC interface. Inaddition, this reflector coating is formulated such that phase shiftupon reflection is minimized.

[0083] A third AR coating is disposed on a first side of the spacerplate, the first side of the spacer plate facing the liquid crystal.This coating minimizes loss by accounting for the index change as lightpasses through the LC, polyamide, conducting layer, glass interface. Afourth AR coating is applied to the on a second side of the spacer plateopposite, its first side. This coating minimizes loss as light passesthe glass, air interface. A fifth coating is applied to the firstsurface of the second substrate (facing the resonant cavity). This is areflector coating forming the second member of the facing Fabry-Perotmirrors. This reflector coating formula minimizes loss at the air,reflector, glass interface, and minimizes phase change upon reflection.Finally, a sixth AR coating, identical to the first AR coating, isapplied to the second (output) side of the second etalon, minimizingloss at the glass-air interface.

[0084] A fourth embodiment of the present invention is represented by amethod of constructing liquid crystal etalons.

[0085] According to one embodiment of the present invention, an LCetalon (or series of LC etalons) coupled to a fiber is used to select asingle WDM channel and route it on a different fiber. The specificspectral channel that is coupled to the output fiber is electronicallyselectable with a switching time of a few milliseconds. With two etalonsin series, one channel among more than 100 channels can be selected.Three etalons in series expands the channels to many hundreds, orthousands.

[0086] One technique to accomplish optical multiplexing is by using anarray of etalons coupled to an input fiber or fiber bundle carrying abroadband signal (e.g. the ITU C-band). Each separate etalon in thearray is free to select a WDM channel independently. This results in adevice that can select a WDM channel from a fiber, while another etalonin the array can select another (or the same) channel. Each etalon inthe array can be tuned to select any prescribed channel, any combinationof channels can be selected by the array—resulting in an all-opticalmultiplexer that is electronically addressable. This WDM embodiment ofthe invention is illustrated in FIGS. 2 through 4.

[0087] Referring to FIG. 2, a 35 channel LC Fabry-Perot WDM deviceaccording to the first embodiment of the present invention isillustrated. Multiple optical fibers, each carrying all wavelengths froma trunk fiber, include a sapphire cylindrical lens at their terminus tocollimate light into the etalon assembly. The cylindrical lenses areepoxied to the surface of the first optical substrate. Each fiber ispositioned above isolated etalon pixels, which are established byetching the conducting layer that is applied to the inside surfaces thatsurround the liquid crystal in each etalon gap. Each pixel isindividually tuned by application of selected electric fields isolatefrom adjacent pixels by breach in the transparent conducting layer.Selected wavelengths from each pixel are passed from the second etalonthrough fibers attached to on the output side. Only 32 pixels areillustrated for clarity, but hundreds are possible. Two of thesedevices, when configured at each end of an optical circuit, form acomplete optical cross-switch.

[0088] Broadband light enters the WDM device through multiple fibers 201emanating from a trunk. (According to an alternate implementation, thelight from the trunk is collimated and displayed across the entireaperture of the device.) Cylindrical collimating lenses 202 are epoxiedto the ends of each of these input fibers, which in turn are attached tothe input surface 203 of the first WDM etalon assembly. There are twoetalons 204, 205 in the WDM device, and each etalon is composed of apair of substrates. Two substrates 204 a, 204 b establish thesuppression etalon 204, and another two substrates 205 a, 205 bestablish the resolving etalon 205. Implementation details of the etalongaps (i.e., material, etc.) are discussed infra with respect to FIGS. 7and 8.

[0089] The etched conducting layer, establishing the square pixels 206is shown on the surface of the second substrate member 204 b, 205 b ofeach etalon. These pixels are also present on the first substrate member204 a, 204 b of each etalon, such that resonant cavities are establishedbetween each pixel pair separated by the respective etalon gap. SpecificAC voltages applied to each pixel independently along wires 207determine the wavelength passed by that pixel, since voltage determinesthe refractive index of the LC within the etalon gap. Because this is atwo etalon series, pixels spatially aligned between the two etalons,e.g. pixels 206 and 208, are tuned electronically to the samewavelength. Ultimately, the light passing out of the device along fibers209 is wavelength sorted according to any prescription, such that aparticular fiber can transmit any resolution element within the bandspass (two fibers can contain the same resolution element), or can betuned off-band to pass no light at all.

[0090] Referring to FIG. 3, a plan view of the WDM device according tothe embodiment shown in FIG. 2 is illustrated. Here, the broadband inputlight arrives along fibers 301, and is collimated for presentation tothe etalons using cylindrical collimating lenses 302. The suppressionetalon 303 has two coated glass substrates 303 a, 303 b and theresolving etalon 304 has two coated glass substrates 304 a, 304 b. Notethat the resonant cavity gap is narrower in resolving etalon 304 than inthe suppression etalon 304. The selected wavelengths are focused bycylindrical lenses 305, which are attached to the output fibers 306along which the output signals propagate.

[0091] Referring to FIG. 4, an end-on view of the WDM device accordingto the embodiment shown in FIG. 2 is illustrated, from a perspectivealong the optical axis. From this perspective, the observer views theWDM device through four glass substrates that make up the twin-etalonsystem. The etched pixels, the optical fiber interface, and theelectronic connections are shown. LC pixels 401 are established byetched conducting layers that are applied to the substrate surfacesfacing the resonant cavities (or alternatively, to glass spacers withinthe resonant cavity). The cylindrical lenses 402 at the ends of inputand output fibers are illustrated by the structures within these squarepixels. Wires 403 leading to each pixel for voltage application are alsoillustrated.

[0092] Optical cross connection devices according to the prior art allowfor only a one to one connection; i.e., one receiving channel can becoupled to only one transmitting channel. In contrast, an etalon etchedinto a monolithic array (or an array of single etalons) according to thesecond embodiment of the present invention is useful to couple onetransmitter to one receiver, or one transmitter to many detectors, ormultiple transmitters to multiple detectors. This is all accomplishedthrough electronic switching, with no mechanical adjustment required.

[0093] Referring to FIG. 5, a schematic view of a two channelcross-switch according to the second embodiment of the present inventionis illustrated. For clarity, two pixels only of the WDM device are shownas two independent transmitters and two independent receivers.Wavelengths are selected from a broadband input trunk fiber to thetransmitters. The transmitters are free to select and transmit anywavelength defined by their independent, electronically tuned LCFabry-Perot resonant cavity. They are free to choose the same, ordifferent wavelengths. Signals exiting the transmitters are subsequentlymixed, and the receivers are tuned to select similar, dissimilar, ornone of the transmitted wavelengths in pre-selected (here, two)channels.

[0094] Although only two channels are illustrated for clarity, manychannels (>100) are possible. Broadband light enters the cross switchalong fibers 501 (or the input may simply be optically displayed acrossthe device). Single element LC Fabry-Perot filters 502 may also be pixelelements 502 of an etched multiple channel WDM filter as previouslydescribed. These elements 502 are electronically tuned to transmitselected wavelengths. These wavelengths are transmitted along fibers 503into a mixer 504, which may be a simple as a bundle of the individualfibers emanating from the transmitters 502. Mixed wavelengths exitingthe mixer 504 are carried along fibers 505, to receiver elements 506.These receiver elements 506 may be implemented as single channel LCFabry-Perot filters, or may be implemented as individual pixels withinan array etched onto a substrate as described previously. Each receiverelement 506 may be tuned to any wavelength of the mixed transmittersignals for ultimate distribution to network clients 507. Receivers 506may be tuned to the same wavelength as any other receiver element, to adifferent wavelength from any other receiver element, or to a wavelengththat is not even a component of the mixed signal from the transmitterelements 502. This embodiment, thus forms a completely flexible,wavelength agile optical cross-switch.

[0095] Referring to FIG. 7, a cross sectional view of a single liquidcrystal-filled Fabry-Perot etalon according to the third embodiment ofthe present invention is illustrated, with an enhanced gap widthimplemented via a hybrid gap of glass and liquid crystal. A first etalonsubstrate 702 and a second etalon substrate 704 are spaced apart fromone another. The etalon substrates 702, 704 are preferably formed offused silica. Low phase shift dielectric reflector layers 710, 708 arecoated onto each of respective opposed faces of the etalon substrates702, 704.

[0096] A spacer plate 718 is disposed between the first and secondetalon substrates 702, 704. Precision-dimensioned spacer beads 722, 724define the spacing between the first etalon substrate 702 and the spacerplate 718. The spacer plate 718 is preferably formed of fused silica, asare the spacer beads 722, 724.

[0097] A first transparent conductor layer 706 is also coated onto thefirst substrate 702, and a second transparent conductor layer 712 iscoated onto the face of the spacer plate 718 facing the first substrate702. The transparent conductor layers 706, 712 are preferably formed ofIndium Tin Oxide (ITO). A preferred proportion of components in the ITOis 4% Tin to 96% Indium Oxide.

[0098] The top coating layers on the first etalon substrate 702, and onthe spacer plate 718 are liquid crystal alignment layers 714, 716. Thealignment layers 714, 716 are formed of polyimide (preferably SE7492polyimide). After coating, the polyimide alignment layers 714, 716 areeach buffed to provide alignment functionality. A liquid crystalmaterial 730 is filled in between the first etalon substrate 702 and thespacer plate 718. E-44 liquid crystal is preferred.

[0099] In the implementation illustrated by FIG. 7, the overall gapbetween the etalon substrate glass plates 702, 704 is augmented byinclusion of the high precision spacer plate 718. This gap augmentationpermits higher spectral resolution measurements than is possible in acell limited in gap width by the practical limit of liquid crystal (LC)thickness. Without the gap augmentation innovation, the largest gapthickness is approximately 100 microns. When this feature of the presentinvention is utilized it has been possible to manufacture gaps as largeas 10 mm. Furthermore, larger gaps are possible. FIG. 6 illustrates aninnovative aspect of the present invention wherein a precision glassspacer plate 718 is laminated to one of the etalon substrates 704,preferably using Norland NOA-68 UV adhesive. The reflector 708 coatingremains beneath that lamination. The side of the spacer plate facing theLC includes the Indium Tin Oxide (ITO) layer 712 followed by a polyamidelayer 716.

[0100] Referring to FIG. 6, a cross sectional view of a single liquidcrystal-filled Fabry-Perot etalon according to the third embodiment ofthe present invention is illustrated, with an enhanced gap widthimplemented via a hybrid gap of air and liquid crystal. A first etalonsubstrate 602 and a second etalon substrate 604 are spaced apart fromone another. The etalon substrates 602, 604 are preferably formed offused silica. Low phase shift dielectric reflector layers 610, 608 arecoated onto each of respective opposed faces of the etalon substrates602, 604.

[0101] A spacer plate 618 is disposed between the first and secondetalon substrates 602, 604. Precision-dimensioned spacer beads 622, 624define the spacing between the first etalon substrate 602 and the spacerplate 618. The spacer plate 618 is preferably formed of fused silica, asare the spacer beads 622, 624.

[0102] Precision spacer posts 642, 644 define spacing dimension betweenthe first and second etalon substrates 602, 604. The spacer posts 642,644 are preferably formed of fused silica, are matched to {fraction(1/4)} wavelength in height, and are flat to {fraction (1/10)}wavelength. The spacer plate 618 is notched to provide clearance for thespacer posts 642, 644.

[0103] A first transparent conductor layer 606 is also coated onto thefirst substrate 602, and a second transparent conductor layer 612 iscoated onto the face of the spacer plate 618 facing the first substrate602. The transparent conductor layers 606, 612 are preferably formed ofIndium Tin Oxide (ITO). A preferred proportion of components in the ITOis 4% Tin to 96% Indium Oxide.

[0104] The top coating layers on the first etalon substrate 602, and onthe spacer plate 618 are liquid crystal alignment layers 614, 616. Thealignment layers 614, 616 are formed of polyimide (preferably SE7492polyimide). After coating, the polyimide alignment layers 614, 616 areeach buffed to provide alignment functionality.

[0105] A liquid crystal material 630 is filled in between the firstetalon substrate 602 and the spacer plate 618. E-44 liquid crystal ispreferred. Thus, the LC cell is bounded by a notched (to accommodate thespacer posts) spacer plate and by one substrate. The spacer-plate andsubstrate on the other side of the LC are preferably held in place as acell by epoxy.

[0106] In the implementation illustrated by FIG. 6, a method providingparticularly large gaps is illustrated. According to thisimplementation, precision spacer posts separate the substrates. Ratherthan laminating the spacer plate to one of the etalon substrates, alarge air gap G is formed.

[0107] It is preferred to use optical materials (spacer balls, spacerplates, and spacer posts) of a precise character to best practice thepresent invention so as to provide preservation of good spectral clarity(finesse) in practicing the third embodiment.

[0108] As mentioned above, one feature of the present invention is theuse of phase matching materials at the interfaces of the materials inthe Fabry-Perot cavity. This feature maximizes the throughput of thedevice.

[0109] Referring to FIG. 8, anti-reflective (AR) phase matching coatingsapplied to the surfaces are illustrated. These coatings minimize Fresnellosses when light passes each interface. Because the Fabry-Perot is aresonant cavity, small losses at each interface, after severalreflections within the gap, can destroy throughput of the device. (Thishas always been a limitation for LC Fabry-Perot devices.) Thephase-matching coating designs include:

[0110] 1) glass-to-air anti-reflection coatings 852, 874 at the outsideof each of the etalon substrates 802, 804;

[0111] 2) a reflector coating 872 disposed on an inside surface of theisolated etalon substrate 804, which incorporates into its designformula passage of light from the glass substrate 804, into air;

[0112] 3) an anti-reflection coating 864 for air-to-glass at theinterface of the spacer plate 818 to the air;

[0113] 4) an anti-reflection coating 862 applied to the LC side of thespacer plate 818 (This coating design formula approximates ITO,polyamide, and LC as thin films at the glass-to-LC interface);

[0114] 5) a reflector coating 854 at the substrate-to-LC interface (Thisreflector coating design approximates ITO as a thin film layer betweenthe reflector coating and the glass substrate, and further approximatespolyamide and LC as thin film layers on the LC side of the reflectorcoating).

[0115] Design recipes for phase matching coatings according toembodiments of the present invention are discussed as follows. Table 1details a layering configuration for an anti-reflective coatingaccording to the present invention. TABLE 1 Material Thickness (nm) ITO12.50 MgF₂ 75.00 ZrO₂ 21.91 MgF₂ 65.31 ZrO₂ 11.93 Nylon 100.00

[0116] Table 2 details a layering configuration for a reflective coatingaccording to the present invention. TABLE 2 Material Thickness (nm) ITO9.09 TiO₂ 64.59 SiO₂ 66.55 TiO₂ 45.95 SiO₂ 110.96 TiO₂ 66.41 SiO₂ 113.07TiO₂ 64.62 SiO₂ 81.06 TiO₂ 42.32 SiO₂ 129.78 TiO₂ 96.10 SiO₂ 124.10 Ag20.00 SiO₂ 129.30 TiO₂ 90.57 SiO₂ 132.01 TiO₂ 83.14 SiO₂ 158.21 TiO₂94.93 Nylon 72.73

[0117] Referring to FIG. 9, a perspective view of etalon componentsaccording to an implementation of the third embodiment of the presentinvention is illustrated. A pair of etalon substrates 910, 920 of asingle LC Fabry-Perot cell 900 is illustrated as being converted intomultiple cells by etching the ITO on the surface of one plate 510 with aNd-YAG laser. A grid pattern 922 is etched in the ITO layer to leave amatrix of isolated ITO cells 924. Each isolated ITO cell 924 isindividually connected to a power supply, so that each LC region beneathparticular ITO cells are subject to individualized electric fields. Inthis way, a single LC Fabry-Perot cell is converted to multiple smallcells, each of which may be tuned to a specified spectral location,independent of other cells. This etching represents our third innovationto improve the flexibility and applicability of LC Fabry-Perot devices.

[0118] Connection of the isolated ITO cells 924 to electrical potentialis accomplished by forming wiring runs on the etched grid 922 along withone or more ground planes. This wiring step in the cell fabricationprocess is accomplished using conventional techniques as are wellunderstood by those of skill in the art.

[0119] A grid pattern is shown in FIG. 9 to illustrate the etch aspectof the present invention, because the squared grid pattern is preferredin order to maximize the number of etalons that may be fit on asubstrate. However, the present invention is not limited to such a shapeor arrangement. For example another advantageous pattern is a wedgepattern. Although the wedge pattern does not make as efficient use ofspace as does the square grid, it makes the task of wiring up theisolated ITO cells easier. Other patterns may also be advantageouslyused.

[0120] Each ITO layer in the preferred embodiment is a 4% tin, 125 nmthick layer recipe, preserving 99% transmission through the layer. Eachpolyamide layer is preferably composed of SE-7492 polyamide,hand-brushed to serve as the LC alignment layer, although otherpolyamide types can be used.

[0121] Referring to FIGS. 10 & 11, the spectral responses for two gapconfigurations of two distinct, twin-etalon WDM device working examplesare illustrated.

[0122] Referring to FIG. 10, the spectral response of a WDM deviceimplementing a two-inch substrate according to a first working exampleis illustrated. In this first working example, the resolving etalon hasa gap of 720 microns, and the suppression etalon has a gap of 51.43microns. The top graph shows the spectral response for the resolvingetalon alone, and the middle graph shows the spectral response for thesuppression etalon alone.

[0123] Placed in series, constructive interference between the twoetalons produces a free spectral range of 30.85 nm in the ITU C-band,and so configured, the WDM device includes more than 770 distinctspectral resolution elements, or WDM channels, each with a FWHM width of0.04 nm. The overall spectral response for the WDM device is shown inthe bottom graph.

[0124] Referring to FIG. 11, the spectral response of a WDM deviceimplementing a three-inch substrate according to a first working exampleis illustrated. In this second working g example, the resolving etalonhas a gap of 360 microns, and the suppression etalon has a gap of 25.71microns. The top graph shows the spectral response for the resolvingetalon alone, and the middle graph shows the spectral response for thesuppression etalon alone.

[0125] Placed in series, constructive interference between the twoetalons produces a free spectral range of 31.47 nm in the ITU C-band,and so configured, the WDM device includes more than 372 distinctspectral resolution elements, or WDM channels, each with a FWHM width of0.08 nm. The overall spectral response for the WDM device is shown inthe bottom graph.

[0126] Etalons embodied according to the present invention can be usedin spectral imaging, spectral remote sensing, laser tuning, andtelecommunications. These etalons can be placed in series and will actas a single etalon with a larger free spectral range. The inventionprovides a LC Fabry-Perot based filter fabricated with an arbitraryspectral resolution and the maximum possible transmission. A spacerplate, optionally in combination with spacer posts, is used to create anetalon of arbitrary gap. This results in spectral resolutions muchhigher than achievable using prior art LC Fabry-Perot etalons. Phasespecific coatings are used to maximize transmission.

[0127] According to an alternate implementation, multiple emitters ortransmitters, each in series with an individual etalon, are used toselect which wavelength from the transmitters is coupled to the outputoptical fiber.

[0128] Construction procedure for making an LC etalon for use in a WDMdevice, according to a fourth embodiment, is described as follows.Construction of an LC etalon begins with a very flat and parallel fusedsilica substrate. Plates matched flat to 150^(th) of a wave of light at633 nm are essential. These substrates are coated with a 12.5 nm thicklayer of Indium Tin-oxide (ITO) (4% tin) that has 99% or bettertransmission at the wavelengths of interest on the flat side. Aftercoating a Nd:YAG laser is used to etch a square grid (e.g. 12×12 pixels)into the ITO. The pattern must include a fine line to the edges to allowfor soldering wires onto the etalon.

[0129] After etching, the etalon plates are coated with a highreflector, low phase variation coating. This coating is designed to havea constant phase even though it is bounded on one side by ITO and on theother by polyimide. Coating recipes for an LC etalon are discussed indetail below.

[0130] After etching the etalon plates are spin-coated with apolyimide-solvent mixture. This is a two step process. The etalon platesare spun and the polyimide mixture is applied, then the angular speed isincreased for 30 seconds to ensure a thin coating of polyimide. Theetalons are then baked at 160 degrees C. for 30 minutes to remove thesolvent. The polyimide coated etalons are then buffed and the two platesare ready to be aligned into an etalon.

[0131] A mixture of optical epoxy in which small, fused silica spacers(3-20 microns thick depending on application) have been mixed is appliedin a 1-2 mm ring around the substrate edges, leaving one or two openingsfor the LC to enter the cavity via vacuum filling or capillary action.The etalons are then placed into a 3-point alignment jig and placed on amonochromatic, Lambertian mercury lamp. Pressure is applied whileobserving the interference pattern, as the imaging device (camera, eye,photodetector) is moved perpendicular to the optical axis theinterference pattern “breathes” expands and contracts, pressure isapplied/removed to minimize this breathing. When the ring pattern is asstable as possible, the epoxy is then hardened with UVB radiation andthe etalon is placed in a vacuum chamber and evacuated to 6 microns. Areservoir of LC-18349 (or other LC's) is placed next to the etalon andthe etalon fills via vacuum filling or capillary action.

[0132] The present invention has been described in terms of preferredembodiments, however, it will be appreciated that various modificationsand improvements may be made to the described embodiments withoutdeparting from the scope of the invention. The scope of the presentinvention is limited only by the appended claims.

What is claimed is:
 1. A liquid crystal Fabry-Perot etalon comprising: afirst substrate coated on a first side with a first transparentconductor layer; a first reflector layer disposed over the firsttransparent conductor layer on the first side of the first substrate; analignment layer disposed over the first transparent conductor layer onthe first side of the first substrate; a spacer plate coated on a firstside with a second transparent conductor layer, the first side of thespacer plate facing the first side of the first substrate, the spacerplate having a second side opposite its first side; plural spacersdisposed between the first substrate and the spacer plate to define apredetermined gap between the first substrate and the spacer plate;liquid crystal filled in between the first substrate and the spacerplate; and a second substrate coated on a first side with a secondreflector layer, the first side of the second substrate facing thesecond side of the spacer plate.
 2. The liquid crystal Fabry-Perotetalon according to claim 1, wherein the second substrate is disposeddirectly against the spacer plate.
 3. An optical wavelength divisionmultiplex device comprising: two or more liquid crystal Fabry-Perotetalons connected together in a series combination, wherein each of theliquid crystal Fabry-Perot etalons comprises: a first substrate coatedon a first side with a first reflector layer; a first transparentconductor layer disposed over the first reflector layer on the firstside of the first substrate; an alignment layer disposed over the firsttransparent conductor layer on the first side of the first substrate; aspacer plate coated on a first side with a second transparent conductorlayer, the first side of the spacer plate facing the first side of thefirst substrate, the spacer plate having a second side opposite itsfirst side; plural spacers disposed between the first substrate and thespacer plate to define a predetermined gap between the first substrateand the spacer plate; liquid crystal filled in between the firstsubstrate and the spacer plate; and a second substrate coated on a firstside with a second reflector layer, the first side of the secondsubstrate facing the second side of the spacer plate.
 4. The opticalwavelength division multiplex device according to claim 3, wherein thesecond substrate of each of the liquid crystal Fabry-Perot etalons isdisposed directly against the spacer plate.
 5. An optical cross-connectcomprising: a pair of optical wavelength division multiplex devicesconnected via an optical network, wherein each of the optical wavelengthdivision multiplex devices comprises: two or more liquid crystalFabry-Perot etalons in series combination, wherein each of the liquidcrystal Fabry-Perot etalons comprises: a first substrate coated on afirst side with a first reflector layer; a first transparent conductorlayer disposed over the first reflector layer on the first side of thefirst substrate; an alignment layer disposed over the first transparentconductor layer on the first side of the first substrate; a spacer platecoated on a first side with a second transparent conductor layer, thefirst side of the spacer plate facing the first side of the firstsubstrate, the spacer plate having a second side opposite its firstside; plural spacers disposed between the first substrate and the spacerplate to define a predetermined gap between the first substrate and thespacer plate; liquid crystal filled in between the first substrate andthe spacer plate; and a second substrate coated on a first side with asecond reflector layer, the first side of the second substrate facingthe second side of the spacer plate.
 6. The optical cross connectaccording to claim 5, wherein the second substrate of each of the liquidcrystal Fabry-Perot etalons is disposed directly against the spacerplate.